The exhaust gases of internal combustion engines, including small engines, are known to contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air.
Small internal combustion engines, usually two-stroke and four-stroke spark ignition engines are used to provide power to a variety of machinery, e.g. gasoline-engine powered lawn mowers, chain saws, leaf blowers, string cutters, motor scooters, motorcycles, mopeds and the like. Such engines provide a severe environment for a catalytic exhaust treatment apparatus. This is because in small engines, the exhaust gas contains a high concentration of unburned fuel and unconsumed oxygen. A catalyst article can be mounted downstream of the engine inside another structure such as a muffler. Examples of catalytic articles mounted inside of mufflers are described in United States Patent Application Publication No. 20040038819, the entire content of which is incorporated herein by reference.
Additionally, the vibration of a two-stroke engine can be three or four times that of a four-stroke engine. For example, vibrational accelerations of 70 G to 90 G (G=gravitational acceleration) at 150 hertz (Hz) have been reported for small engines. The harsh vibration and exhaust gas temperature conditions associated with small engines lead to several modes of failure in the exhaust gas catalytic treatment apparatus, including failure of the mounting structure by which a catalyst article is secured in the apparatus and consequential damage or destruction of the catalyst article due to the mechanical vibration and to flow fluctuation of the exhaust gas under high temperature conditions. The catalyst article usually comprises a ceramic-like carrier that has a plurality of fine parallel gas flow passages extending therethrough (sometimes referred to as a “honeycomb”) and which is typically made of e.g., cordierite, mullite, etc., on which a catalytic materials is coated. The ceramic-like material is subject to cracking and pulverization by excessive vibration and exposure to extremely high space velocities (i.e., the amount of air flowing through the catalyst article, which may be 400-500 K or higher). While ceramic and metal monolithic honeycomb catalysts are known to be used in small engine applications, it is desirable to have alternative designs which are adapted to the smaller space, extreme operating conditions and lower overall cost of small engines. In such cases, metal carriers such as metal plates and metal wire mesh have been used. Although metal wire mesh can be easily adapted to small spaces and is relatively inexpensive its flexibility makes it prone to degradation of the catalytic layer under the extreme vibration and air flow conditions of a small engine, thus shortening the useful life of the catalyst.
Catalysts useful in small engine applications are described in United States Patent Application Publication No. 20060171866, the entire content of which is hereby incorporated by reference. Briefly, such catalysts comprise one or more platinum group metal compounds or complexes which can be on a suitable support material. The term “compound” means any compound, complex or the like of a catalytic component which, upon calcinations or use of the catalyst, decomposes or otherwise converts to a catalytically active form, which is often an oxide or metal. Various compounds or complexes of one or more catalytic components may be dissolved or suspended in any liquid which will wet or impregnate the support material.
Three-way conversion (TWC) catalysts have utility in a number of fields including the treatment of exhaust gas streams from internal combustion engines, such as automobile, truck and other gasoline-fueled engines. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum, palladium, rhodium, rhenium and iridium) disposed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The TWC catalyst carrier may also be a wire mesh, typically a metal wire mesh, which is particularly useful in small engines. TWC catalysts can be manufactured in many ways. U.S. Pat. No. 6,478,874, for example, sets forth a system for catalytic coating of a substrate. Details of a TWC catalyst are found in, for example, U.S. Pat. Nos. 4,714,694 and 4,923,842. U.S. Pat. Nos. 5,057,483; 5,597,771; 7,022,646; and WO95/35152 disclose TWC catalysts having two layers with precious metals. U.S. Pat. No. 6,764,665 discloses a TWC catalyst having three layers, including a palladium layer having substantially no oxygen storage components. U.S. Pat. No. 5,898,014 discloses catalyst compositions containing oxygen storage components.
Refractory metal oxides such as alumina, bulk ceria, zirconia, alpha alumina and other materials are known for use as a support for the catalytic components of a catalyst article. The alumina support materials, also referred to as “gamma alumina” or “activated alumina,” typically exhibit a BET surface area in excess of 60 square meters per gram (“m2/g”), often up to about 200 m2/g or higher. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. Although many of the other refractory metal oxide supports suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, that disadvantage tends to be offset by a greater durability of the resulting catalyst.
In an operating engine, exhaust gas temperatures can reach 600° C. and catalyst out temperatures can exceed 1000° C. Such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or rare earth metal oxides, such as ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al., U.S. Pat. No. 4,171,288, the entire content of which is incorporated herein by reference.
Of the platinum group metals, palladium (Pd) is of particular interest for gasoline engine emission control because of its lower cost relative to platinum (Pt) and rhodium (Rh), its greater availability relative to platinum and its performance advantages relative to other platinum group metals under certain operating conditions. However, in spite of price and availability advantages, there are several problems associated with the use of palladium as the only catalytic material in catalyst articles. Palladium is less resistant to poisoning by fuel and motor oil contaminants than platinum. It is also inferior to platinum in its ability to convert short chain saturated hydrocarbons such as ethane and propane. These disadvantages are partially off-set by the durability of palladium, i.e., it is more resistant to sintering than platinum. Nevertheless, the cost advantages of a palladium-only catalyst article are particularly important for meeting exhaust treatment requirements in the less expensive types of machines that incorporate small engines. There is still a need for a palladium-only catalyst article with improved durability and performance in the harsh environment of the small engine. The present invention addresses this need.